Compression, stretching, tearing, or crushing during vaginal delivery may cause severe damage to the pelvic floor tissues and muscles. It is reported that ~55% of pregnant women have symptoms of UI. While ~20-45% of this population experience persistent SUI after vaginal delivery,7–11 the remaining population is still at risk to develop incontinence later in life.12–16 Particularly, pudendal nerve (PN) entrapment and injury after vaginal delivery may result in decreased expression of regenerative cytokines and neurotrophins (i.e., brain-derived neurotrophic factor (BDNF)) followed by denervation of the external urethral sphincter (EUS), causing SUI.17–22
It was shown that direct electrical stimulation (ES) of PN using wired metal electrodes can provide regeneration via BDNF secretion of injured neurons.17–23 However, this strategy has not transitioned into the clinic due to three main challenges: (1) ES is applied intraoperatively only once (for ~1 hour) during the surgery. (2) Postoperative ES is difficult to apply due to invasive and blind penetration of electrodes to the PN site lacking precise, accurate, and local treatment. (3) ES via metal electrodes requires professional personnel and does not allow on-demand self-treatment for patients, impeding practical clinical or home treatment.
Our objective is to develop a piezoelectric material-based biodegradable and implantable platform that can provide wireless, local, and on-demand mechano-electrical stimulation (MES) of PN upon applied mechanical forces via exercise movements for postoperative SUI treatment. Our hypothesis is that local MES, provided upon mechanical forces generated by exercise movements on the implanted piezoelectric platform around the PN, can activate neurons, restore downregulated neurotrophins and promote regeneration and reinnervation for SUI treatment.
For this purpose, we developed a flexible cuff electrode-integrated piezoelectric platform. The conductive graphene based flexible electrode cuff was already developed and characterized using our patented approach (US Patent 11,938,708 & US Patent 11,926,524). The biodegradable poly-L-lactic acid (PLLA), chitosan (CS) and whey protein isolate (WPI)-based piezoelectric part was fabricated using electric field assisted polymer casting and the whole system was integrated via layer-by-layer assembly.
The structure and integrity of the polymer layers in the platform was assessed by taking surface and cross-section images in scanning electron microscopy (SEM). The crystal transformations and structural characteristics was determined by X-ray diffraction (XRD) analysis. Mechanical properties of the piezoelectric platform were determined via tensile testing to assess the stiffness and flexibility. We also determined the 3D topographical surface map of the platform using high resolution stylus profilometer and 3D optical microscope. The thickness, crystallinity and roughness of each layer was determined by using spectroscopic ellipsometer. The conductivity and sheet resistance of the interdigitated graphene electrodes was evaluated using 4-point probe measurement. The piezoelectric response of the platforms prepared under different processing conditions was evaluated by measuring the output voltage and current upon applying a constant external force (i.e., 1-10 kPa at a frequency of 1-20 Hz) using a custom made linear motor set up. The stability and in vitro degradation of the platform along with the swelling ratio was also measured. The in vitro biocompatibility of the platform was tested on human motor neurons, Schwann cells, skeletal muscle cells and fibroblast cells via the standard live-dead cell assay, cell cycle, apoptosis, DNA damage and ROS generation tests. In vitro therapeutic efficacy of MES applied through the piezoelectric platform was tested on motor neurons and Schwann cells co-culture. The neurite extension along with the expression of neuronal and glial markers was also assessed using ICC, ELISA, RT-PCR and Western Blot assays. The implantation surgery feasibility of the device was initially tested on cadaver animals. We also tested the in vivo biocompatibility of the device on Sprague Dawley rats.
Our results indicated that the developed platform showed stability and integrity under different degradation conditions by maintaining its structure and electrical properties. The platform generated enough voltage output (50-150 mV) upon applied external force (1-10 kPa at a frequency of 1-20 Hz- mimicking the movements of the rat) to stimulate the cells and tissues for regeneration purposes. The platform materials as well as the applied MES conditions did not cause any toxicity, cell death, DNA damage, apoptosis, ROS or alteration in the cell cycle showing its in vitro biocompatibility. The functional assays indicated that the applied MES through the piezoelectric platform resulted in enhanced neurite extension along with significant expression of neuronal and glial markers, demonstrating the regenerative potential of the device. The designed device was easy to handle for in vivo implantation and suturing on SUI model. Basically, the flexible electrode cuff was wrapped around the PN while the connected piezoelectric platform was subcutaneously placed on the right hindlimb muscle of the rats. Our initial in vivo biocompatibility tests showed no adverse effect of the platform. Currently we are working on testing the in vivo regeneration potential of the developed device.
In conclusion, this innovatively engineered biodegradable and implantable piezoelectric platform holds the potential to enable local, wireless, and postoperative MES of PN. In the long term, this piezoelectric platform can be synergistically combined with Kegel exercise to provide local MES upon rehabilitative Kegel movements to promote PN regeneration and reinnervation